The invention relates to a drive mechanism for radial adjustment of a drilling tool in a drill head by means of an axially moveable drive rod which is connected to a rotary drive motor via a screw drive mechanism.
The radial adjustment of a drilling tool in a drill head has the purpose of separating the drilling tool from the surface being cut, at the conclusion of a drilling operation, in such a way that no scores or grooves are left [i.e., produced] in said surface. The mechanism also serves to produce a radial adjustment of the drilling tool to compensate for cutting wear occurring on the tool. This compensation enables the operating life of a drilling tool to be greatly extended, because in general the limiting condition in tool life is the point where, due to cutting wear, the dimensions of the surface being cut are no longer within the acceptable range.
Most types of mechanism for radial adjustment of a drilling tool in a drill head have the feature that a drive rod, which is centrally disposed in the drill head and extends through the tool spindle holding which spindle holds the drill head, acts directly or indirectly on the drilling tool via an inclined surface, whereby the tool is radially adjusted. The drive rod rotates with the drill head and the tool spindle. The drive rod is driven axially by a rotary drive motor via a screw drive which converts the rotational movement produced by the motor to an axial thrust movement.
In a known drive mechanism of the type initially described above (Ger. Auslegeschrift 21 67 082), the rotary drive motor is connected to the screw shaft via a rotating coupling which permits longitudinal movement. The internal screw which engages the screw shaft is fixed to the mounting. The end of the screw shaft which is distant from the rotary drive motor is connected to the drive rod via a thrust bearing.
There are embodiments of the type of drive mechanism described initially above in which the rotary drive motor is not aligned with the longitudinal axis of the screw shaft but with an axis intersecting the longitudinal axis (U.S. Pat No. 3,286,556) or with an axis parallel to the longitudinal axis (Fr. Pat. No. 2,327,840). In contrast to these, the known device of Ger. AS 21 67 082 (cited supra) has relatively small lateral dimensions (diameter); however, the axial length is relatively large. A significant factor in this is the fact that the achievable axial thrust movement is reflected twofold in the axial length of the drive mechanism--first in the rotating coupling, in which allowances must be made for this axial movement, and secondly in the length of the screw shaft which extends beyond the internal screw, which length also must equal the achievable axial movement. Further, there is a degradation of the adjustment accuracy of the known drive mechanism due to the fact that there is play in the required rotating coupling, at least when unavoidable wear develops.
An object of the invention is thus to devise a drive mechanism of the type described initially above, such that the mechanism has same-axis construction, and thereby small lateral dimensions, but is also as short as possible in its axial dimension, and further is comprised of types of elements which in combination result in very high tool adjustment accuracy.
This object and others are achieved according to the invention in that a screw shaft which is connected to and disposed on the same axis as the output shaft of the rotary drive motor is rotatably mounted in an axial bearing in a housing, and engages an internal screw element. The internal screw element is fixed to a thrust body which is axially displaceable in the housing. A rotatable drive rod which can be connected to the thrust bar in the tool is rotatably mounted in an axial bearing in the thrust body. The free end of the screw shaft extends into a bore hole in the end of the drive rod.
The types of working elements used, namely axial bearings and a screw drive, make possible an embodiment of the drive mechanism with very high adjustment accuracy. The axial dimension is relatively short, due to the fact that the necessary axial adjustment distance is reflected only once in the axial dimension of the drive mechanism, namely at the location where it is necessary to provide room for the axial movement of the thrust body. Although the part of the screw shaft which projects from the internal screw must also be at least as long as the prescribed displacement distance for effecting the adjustment, in the inventive mechanism it does not add to the overall axial dimension of the drive mechanism because the free end of the screw shaft extends into the bore hole in the end of the drive rod.
In a particularly advantageous embodiment of the invention the axial bearing of the drive rod is disposed axially adjacent to the internal screw element and surrounds the end of the drive rod. In this way the axial segment of the mechanism in which the free end of the screw shaft extends into the bore hole of the drive rod is also simultaneously employed to accommodate the the axial bearing for the drive rod, so that no additional axial space is taken up in mounting this axial bearing. The sole condition which determines the length of the thrust body is the need to dispose the internal screw element and the axial bearing for the drive rod axially adjacent to each other.
A particularly advantageous embodiment of the invention from the manufacturing engineering standpoint is characterized in that the thrust body is axially displaceably guided in a bore in the housing. The axial bearing of the screw shaft is mounted in an insert which is held in the bore in the housing. Under this arrangement the housing comprises essentially a cylinder with a longitudinal bore, which can be manufactured with very high precision. The thrust body is guided in the longitudinal bore, and also the axial bearing of the screw shaft is held in an insert which is mounted in the bore. Thus in addition the arrangement greatly simplifies the assembly of the drive mechanism.